Not sure what you mean by Gainsight Plugin?
They have a similar tool called Co Pilot that replaces MSI, effectively. GS would prefer you use them instead of Marketo, so be careful!
Usually people use GS with Co Pilot for specific kinds of communications where their database can handle scenarios like Usage, Secure Comms, etc... whereas Marketo is used for larger scale programs that don't involve salespeople.
I find Co Pilot is a lot like Outreach and doesn't have some of the nice things Engagements can do, but that's not really how it's supposed to be used.